gpg: Fix out-of-bound read in subpacket enumeration
983f7b2acbd1
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Description

gpg: Fix out-of-bound read in subpacket enumeration

* g10/parse-packet.c (enum_sig_subpkt): Check buflen before reading
the type octet.  Print diagnostic.

If the final subpacket has only a length header evaluating to zero and
missing the type octet, a read could happen right behind the buffer.
Valgrind detected this. Fix is obvious. Note that the further
parsing of the subpacket is still okay because it always checks the
length. Note further that --list-packets uses a different code path
and already reported an error.

He provided a test file copied below. Running "gpg -v --verify" on it
triggered the bug.

-----BEGIN PGP ARMORED FILE-----
Comment: Use "gpg --dearmor" for unpacking

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MSwgMHhjMSwgMHgyLCAweDEsIDB4MiwgMHg3LCAweDQwLCAweDIsIDB4MiwgMHgy
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LCAweDIsIDB4MiwgMHgyLCAweDIsIDB4MiwgMHgyLCAweDIsIDB4MiwgMHgyLCAw
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MHgyLCAweDIsIDB4MiwgMHgyLCAweDc3LCAweDg3LCAweDIsIDB4MiwgMHgyLCAw
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